1. Field of Invention
The present invention relates to a light emitting device driver circuit and a driving method of a light emitting device circuit; particularly, it relates to such light emitting device driver circuit with total harmonic distortion (THD) compensation and a method for driving such a light emitting device circuit.
2. Description of Related Art
FIG. 1A shows a schematic diagram of a prior art light emitting diode (LED) driver circuit 10 and its related circuits. As shown in FIG. 1A, the LED driver circuit 10 includes a switch circuit 11, a switch control circuit 12, and a constant current source 13. The LED driver circuit 10 drives an LED circuit 20. The LED circuit 20 includes plural LEDs connected in series, and the plural LEDs are divided into plural LED groups, e.g. four LED groups G1, G2, G3, and G4 as shown in FIG. 1A. The switch circuit 11 includes plural switches (e.g. four switches S1, S2, S3, and S4 as shown in FIG. 1A), which are electrically connected to the corresponding LED groups G1, G2, G3, and G4 respectively. A rectifier circuit 30 rectifies an AC voltage provided by an AC power source 40, to generate a rectified input voltage Vin as shown in FIG. 1B. The LED driver circuit 10 drives the LED circuit 20 by turning ON or OFF the switches S1-S4 according to the level of the rectified input voltage Vin, whereby one or more of the LED groups G1-G4 glow accordingly.
For example, as shown by the signal waveforms in FIG. 1B, when the level of the rectified input voltage Vin is lower than level L1, the switch control circuit 12 turns OFF all the switches S1-S4. When the level of the rectified input voltage Vin is between levels L1 and L2, the switch control circuit 12 turns ON the switch S1, and turns OFF the switches S2-S4, whereby the LED group G1 glows. Similarly, when the level of the rectified input voltage Vin is between levels L2 and L3, the switch S2 is turned ON, and the switches S1 and S3-S4 are turned OFF, whereby the LED groups G1-G2 glow. Similarly, when the level of the rectified input voltage Vin is between levels L3 and L4, the switch S3 is turned ON, and the switches S1-S2 and S4 are turned OFF, whereby the LED groups G1-G3 glow. And when the level of the rectified input voltage Vin exceeds level L4, the switch S4 is turned ON, and the switches S1-S3 are turned OFF, whereby the LED groups G1-G4 glow. U.S. Pat. No. 6,989,807, U.S. Pat. No. 7,081,722 and US 2011/0273102 are relevant prior art patents for reference.
In the prior art illustrated by FIGS. 1A and 1B, the constant current source 13 provides a constant current, that is, when one or more of the LED groups G1-G4 glow, the current I1 flowing through the conductive LED(s) is constant. Referring to the signal waveform of the current I1 as shown in FIG. 1B, regardless how many LED groups of the LED groups G1-G4 glow, the current flowing through the conductive LED(s) is the constant current. Only when the level of the rectified input voltage Vin is lower than the level L1, i.e., when all the switches S1-S4 are turned OFF, the current I1 is zero current.
In comparison with a conventional driver circuit which drives the LED circuit by a DC voltage, the prior art LED driver circuit 10 has an advantage that the manufacturing cost of the LED driver circuit 10 is relatively lower because it does not need to convert the rectified input voltage to a DC voltage. If the rectified input voltage has a frequency which is enough high, naked eyes will not perceive any flicker of the LED circuit 20. However, the prior art LED driver circuit 10 has a disadvantage that a total harmonic distortion (THD) of the prior art LED driver circuit 10 is high.
In view of above, the present invention proposes a light emitting device driver circuit with THD compensation and a method for driving a light emitting device circuit.